Colombo: Gunmen opened fire at an opposition rally in Sri Lanka Saturday, marking an escalation of violence ahead of next week`s crucial presidential elections, party officials and police said.

Unidentified attackers fired from a jeep as the main opposition candidate Maithripala Sirisena was leaving the meeting in his home constituency of Polonnaruwa district, his office said.

A party official said Sirisena was unhurt but a bystander was injured, while a vehicle was also damaged. Police sources said an investigation was underway, but no arrests had been made.

The latest attack, by far the worst targeting the opposition in the run-up to the January 8 election, came a day after a stone-throwing attack injured supporters of Sirisena elsewhere in the island.

Campaign-related violence has escalated across Sri Lanka as President Mahinda Rajapakse fights an unexpectedly tough battle to remain in power, poll monitors said.

The private Campaign for Free and Fair Elections (CaFFE) monitoring group said it had received 1,073 complaints since campaigning got underway in early December.

Police said they had received a much smaller number of complaints, but that 130 arrests had been made.

A deputy minister and 13 other elected representatives are among those who have been arrested in connection with 245 complaints of violence, police spokesman Ajith Rohana said.

CaFFE said the increasing violence was well organised and target the opposition, which is mounting a serious challenge against two-term incumbent Rajapakse.

"We are seeing a trend of increasing violence," CaFFE director Keerthi Tennakoon told AFP. "The violence is well organised. It is almost always directed against the opposition`s campaign."

Rajapakse`s main rival Sirisena had a narrow escape when stones and rocks were thrown at a political rally outside Colombo Friday night, but some 20 of his supporters were injured, the party said.

Police spokesman Rohana said two men had been arrested in connection with Friday`s stone throwing and they were looking for four more suspects.

Sirisena blamed pro-government elements for unleashing the attack on his rally.

Sri Lanka`s External Affairs ministry on Friday warned the European Union not to interfere in elections after Colombo-based ambassadors urged the government to ensure peaceful polls -- a sentiment echoed by UN chief Ban Ki-moon in a separate statement earlier this week.

Previous elections have been marred by violence, particularly against Tamil and Muslim voters, the opposition`s major support base.

Sirisena left his health portfolio and quit Rajapakse`s administration in November to become the main opposition candidate.

Local media reports say that the two men are currently neck and neck, although Rajapakse was considered the clear front-runner when he called the vote two years ahead of schedule.

The ruling party`s vote sank 21 points at a local election in September, suggesting that the president`s own popularity may be waning five years after he was credited with ending a separatist war that had claimed 100,000 lives between 1972 and 2009.